Apartment houses are using a community receiving system having a community antenna and in which broadcast signals received by the community antenna are redelivered to each household. It is expected that the community receiving system delivers analog signals of a ground wave broadcast in the VHF (very-high frequency) band and in the UHF (ultra-high frequency) band, as well as digital broadcast signals of a broadcast satellite (BS) and a communication satellite (CS).
In redelivering the signals by the community receiving system, the bands of intermediate-frequency signals are overlapped between the BS broadcast and the CS broadcast, making it impossible to transmit the signals through one cable by simply mixing all signals together. If a cable is separately laid to transmit the signals of the CS broadcast, the construction of a large scale is necessary and, besides, an additional device such as a switch for changing over the cables must be attached to the receiver unit of each household. Therefore, there has been proposed a modulation conversion system which converts the system for modulating the signals of CS broadcast, shifts the frequency into an empty channel and transmits the signals through one cable (see, for example, JP-A-2001-245175 (FIG. 1)).
When the modulation conversion system is employed for the community receiving system, however, the channel is converted into a channel of a frequency different from that of the channel used for the broadcast and, hence, the frequency data contained in the broadcast signals become no longer in agreement with the frequency of signals really transmitted through the cable. As an example of digital signals, the transport stream signals of an MPEG (moving picture coding experts group) 2 contain frequency data as control data related to the network, and the receiving operation is carried out based on the frequency data. Therefore, the frequency data in the network information table must be corresponded to the frequency on the cable.
As a technology for automatically rendering the frequency to be corresponded on the receiver unit side, there has been proposed a technology for automatically forming, for example, a conversion table representing a correspondence between the frequency data in the network information table in the ground wave digital broadcast and the frequency on the cable on the receiver unit side (see, for example, JP-A-2001-275011 (FIG. 7)). According to this prior art, it is judged whether the signals can be received while successively setting the frequency in the frequency search mode, and a frequency conversion table is formed by rendering the frequency data of the network information table of when the signals are received to be corresponded to the frequency that has been set.
According to the above prior art, the frequency conversion table is formed by successively setting the frequency, and by rendering the frequency data of the network information table to be corresponded to the frequency that has been set. This is based on a prerequisite in that in the ground wave digital broadcast, the network and the channel (transport stream) have been corresponded in a one-to-one manner, and the network information table that is obtained contains a sole frequency data only.
In the CS broadcast, however, a plurality of transport streams are fed from a satellite (network) while the same network identifier is attached to the transport streams from the same satellite. Therefore, even if the network information table is obtained by setting the frequency, a plurality of frequency data are contained in a random fashion therein, and it is not allowed to readily judge which frequency data is corresponded to the frequency that has been set.